The χ² hypersurface (in red) shown in a space of calibration and image models. The current solution is shown as the point with coordinates ﹩({{\mathsf{M}}}_{o}^{},{I}_{o})﹩, where ﹩{{\mathsf{M}}}_{o}^{}﹩ and I_o represent the starting calibration and image model, respectively. The arrows overlaid on the contour plot show successive application of the calibration and imaging solvers to iteratively move the solution to minimize the metric in Equation (5).